Our studies have identified chemicals which are excreted into milk in greater concentrations than predicted by established pharmacokinetics principals. Our current work suggests that active/facilitated secretion mechanisms may be responsible for the unpredictable milk-plasma ratios that are observed. We are working to characterize the mechanisms responsible for the non-diffusion mode of xenobiotic accumulation in experimental models and to explore the relevance of these observations in humans. Established in vivo and in vitro models are being used to identify other potential substrates for facilitated/active uptake by mammary secretory cells and to characterize the nature of the transport mechanisms. This work can be expanded to include human studies to provide better quantitative estimates of the propensity for chemicals to be transported to nursing infants.